(1) Field of the Invention
The present invention relates to a paper-like product useful as a fiber reinforcement for fiber-reinforced plastics (hereinafter referred to as "FRP") and having a special structure, a prepreg formed by impregnating the paper-like product with a matrix resin, and a method and apparatus for the production of the paper-like product.
(2) Description of the Related Art
For the fiber orientation in FRP, it is preferred that the fibers be oriented in the direction of a force given to FRP or a stress produced in FRP. Accordingly, a material in which continuous filaments are arranged in one direction, that is, a unidirectional (UD) material, has been considered the best fiber reinforcement for FRP. This material is most preferred as a material for producing a feature that fibers are oriented in the direction of a stress produced in FRP. However, it is difficult to prepare a fiber reinforcement in the form of a thin sheet from this UD material with any economic advantage. More specifically, a filament strand used as a fiber reinforcement for FRP is generally thick in size, and it is very difficult to arrange filaments of this strand properly in one direction and spread the strand into a sheet form.
One necessary function of a fiber reinforcement in the form of a thin sheet is a reinforcing action in the direction perpendicular to the orientation direction in the UD material. For example, there can be mentioned a reinforcing action in the circumferential direction in a pipe-like product. In the case of a pipe-like product, for example, a fishing rod, a UD material is arranged so that its orientation direction is in agreement with the longitudinal direction, and in order to improve the flexural strength, fibers are also arranged properly in the circumferential direction, that is, in the direction at a right angle to the orientation direction of the UD material (hereinafter referred to as "90.degree. reinforcement"). A small amount of fibers is sufficient for this 90.degree. reinforcement, and generally, this amount is less than 10% of the amount of fibers arranged in the orientation direction of the UD material (hereinafter referred to "0.degree. reinforcement"), though this amount may be varied to some extent according to the desired end product.
Furthermore, a sheet material which is as thin as possible and has a good dimensional stability is desirable as a reinforcement for a base of a printed circuit board.
A paper-like product formed as an aggregate of staple fibers is a material suitable for obtaining a thin sheet-like product. However, in conventional paper-like products, fibers are oriented substantially isotropically, and in view of the above-mentioned conventional idea that fibers should be properly arranged in the direction of the stress produced, these paper-like products are not preferred.
The present inventors carried out research into the relationships of the fiber orientation in a paper-like product to the physical properties of FRP obtained by impregnating this paper-like product with a matrix resin. At first, carbon staple fibers having a diameter of 8 .mu.m and a length of about 10 mm, as a typical example of the conventional fiber reinforcement, were dispersed at a concentration of 0.05% by weight in an aqueous 0.4% solution of carboxymethyl cellulose. The resulting dispersion (hereinafter referred to as "slurry") was stirred by a rotary vane. Various orientation states of carbon staple fibers in the slurry could be obtained by conducting stirring under various stirring conditions. In an enlarged photograph of paper obtained from these oriented staple fibers by the hand paper making method, the number of staple fibers oriented within .+-.5.degree. to a predetermined direction was counted, and the ratio (%) of these staple fibers to all the staple fibers in the paper-like product was calculated as the "orientation degree". Namely, the orientation degree is expressed by the following formula: ##EQU2##
The obtained paper-like product was impregnated with about 150% by weight of an epoxy resin as a matrix resin and the resin was cured to obtain plate-like FRP reinforced with the paper-like product. The tensile strength (T.sub.0) in a direction of orientation of the staple fibers and the tensile strength (T.sub.90) in a direction perpendicular to the orientation direction were determined, and the ratio T.sub.0 /T.sub.90 was calculated. The obtained results are shown in Table 1.
TABLE 1 ______________________________________ Orientation degree (%) T.sub.0 /T.sub.90 Remarks ______________________________________ 6 1.2 Substantially isotropic 43 11.7 63 19.3 81 23.8 90 25.4 98 24.6 Substantially unidirec- tionally oriented ______________________________________
From the results shown in Table 1, it is seen that the orientation effect is prominent when the orientation degree is at least 50%. The orientation degree is preferably at least 60%, and more preferably, at least 80%. Namely, it has been confirmed that a paper-like product in which the content of staple fibers oriented in a specific direction is at least 50% is valuable as a reinforcement. This can be confirmed in not only carbon fibers but also in alumina fibers, glass fibers, and cellulose fibers. Moreover, this can be confirmed in paper-like products composed of whiskers and those composed of at least 2 kinds of staple fibers.
The above-mentioned paper-like product, that is, a paper-like product, especially a thin paper-like product having a basis weight of not more than 20 g/m.sup.2 in which the content of staple fibers oriented in a specific direction is at least 50%, involves problems with respect to handling thereof.
When the fiber orientation direction is in agreement with the longitudinal direction of the paper-like product, handling of the paper-like product is relatively easy. However, in the case of a paper-like product in which the orientation direction is perpendicular to the longitudinal direction, handling must be carried out very carefully, because the paper-like product is readily broken by a very small force. This tendency is especially prominent in a paper-like product having a small basis weight, although the use of the thin paper-like product is advantageous. Accordingly, in a paper-like product having fibers oriented in a specific direction, one of the important problems is how to improve the strength in the direction perpendicular to the fiber orientation direction. Namely, there has been the technical problem of how to improve the strength in the direction perpendicular in the orientation direction with a small amount of fibers while orienting reinforcing fibers in a certain direction as much as possible. As means for solving this problem, there have been adopted a method in which the amount of an adhesive as a constituent of the paper-like product is increased or a method in which the same or different kinds of paper-like products are laminated so that their orientation directions are perpendicular to each other. The former method has a problem in that the adhesive has an adverse influence on the physical properties of the final product, especially the adhesion between the fiber and resin. The latter method has a problem in that a thin paper-like product cannot be obtained.